Many security documents, such as bank notes, include different security elements including security dyes, security inks and security threads. These security elements typically have magnetic and (or) conducting properties which can be detected. Counterfeited documents sometimes have magnetic properties in those regions, where an authentic document has not any magnetic properties and therefore detection of these elements and the location is useful to determine whether a banknote is authentic.
There are various types of induction sensors, which are able to detect magnetic and conductive properties of documents. When a document, having magnetic or conductive properties, is moved past an induction sensor, it causes changes in the inductiveness and magnification factor of the induction sensor as inductance. Electronic circuitry associated with the induction sensor detects changes in the properties of the induction sensors.
Most induction sensors contain ferromagnetic cores for forming a high magnitude magnetic field on the document under test. Magnetic properties of such cores depend on the magnitude of the external magnetic field. When an induction sensor is situated in an external magnetic field with time-varying magnitude the signal from the sensor may be undistinguishable from a signal associated with passing of security document. Wide dispersion of magnetic permeability of ferromagnetic cores leads to wide dispersion of initial inductance of such types of induction sensors. Due to this dispersion complicated electronic circuitry or individual adjusting elements are often required.
In many induction sensors, the exciting coil is made from wire coils, containing many turns on a special former. Such coils and other elements of this type of induction sensors are expensive to manufacture. Induction sensors with such wire coils often have a wide dispersion of inductance, even if the sensors have ferrite cores.
Most induction sensors have a small geometrical vicinity for sensing. In banknote validators using such types of induction sensors special mechanical arrangements are used to hold the banknote in close proximity to or in direct contact with the induction sensor. Such an arrangement leads to a high probability of bill jamming and to high wear of the induction head.
Some induction sensors provide testing of documents without direct contact with the documents due to placement of two inductive parts of the same induction sensor on opposite sides of a validating channel. The main problem with this approach is the two induction parts are connected by relatively long wires which carry a high frequency signal. These wires add a source of interference.
The present invention departs from the accepted approach of using wire coils and ferromagnetic cores as are common in prior art. The present induction sensor can be manufactured using a low cost multilayer printed circuit board technology and provides a narrow dispersion of induction sensor parameters. The present induction sensor can be manufactured with various resolution sensing zone parameters and has particular application in banknote validators and without direct contact of the banknote. Analysis of signals from two such induction sensors with individual associated electronic circuits placed on opposite sides of validator channel reduces variation of the signal as a function of the distance between the sensor and the banknote.